Solid/Start CreateAsync/createResource Mixes Results Server-side: Bug & Fixes

Jul 25, 2025 by James Vasile 78 views

Solid/Start: `createAsync`/`createResource` Mixes Results on Server Side Due to Unstable Component Execution

Introduction

Hey guys! Today, we're diving deep into a peculiar bug in SolidJS and SolidStart that can cause some head-scratching moments when dealing with server-side rendering. Specifically, we're going to explore an issue where createAsync and createResource can mix up results when your component's execution is a bit… unstable. This happens due to secondary reruns of the suspended component and incorrect mapping of asynchronous data. So, let's get into it and see how we can tackle this! This article will thoroughly explain the SolidJS server-side rendering bug, focusing on how createAsync and createResource can lead to unexpected behavior. We'll break down the technical details, provide a clear example, and discuss potential solutions to ensure your SolidJS applications run smoothly on both the client and server.

The Bug: A Closer Look

So, what's the fuss all about? Imagine you're building a SolidJS app with server-side rendering (SSR). You're using createAsync or createResource to fetch data, which is pretty standard. But, under certain conditions, the results from these asynchronous operations can get mixed up, leading to incorrect data being displayed. This bug primarily occurs when the component has what we call "unstable" execution. In essence, this means that the component gets rerun, specifically when SolidJS attempts to match results based on the initial order of requests. Let's make sure you guys understand this: the main reason behind the bug is that the component reruns during the server-side rendering process. This can happen due to various reasons, such as redirects or changes in application state. When the component reruns, SolidJS might not correctly map the results of asynchronous operations, especially if the order of these operations changes between the initial render and the rerun. This is where the createAsync and createResource functions come into play. These functions are designed to handle asynchronous data fetching, but they rely on a consistent execution context to ensure the correct mapping of results. When the execution order becomes unstable, these functions can misattribute the fetched data, leading to the mixed-up results we're trying to avoid. To understand this better, let's delve into an example that showcases the issue. We'll see how a simple redirect can trigger the bug and how the order of createAsync calls can influence the outcome. By examining this example, we can gain a clearer picture of the problem and start thinking about how to fix it.

Example: Reproducing the Issue

To illustrate this bug, let's walk through a practical example. We have a StackBlitz setup that you can play around with to see the issue in action. The core of the problem lies in a component that might redirect based on some condition. When this redirection happens server-side, it can trigger the secondary rerun we talked about.

Steps to Reproduce

Go directly to the /about route in your browser (don't use a link within the app to navigate).
Open your browser's console and observe the logs.

You'll notice that the expected output doesn't match what you actually see. The values from the asynchronous operations aAsy and bAsy are mixed up.

Expected vs. Actual Behavior

Here's what we expect to see:

if (shouldRedirect()) {
 console.log('LATER');
 console.log('A:', aAsy()); // expected "A: a" actual "A: b"
 console.log('B:', bAsy()); // expected "B: b" actual "B: a"
} else {
 console.log('INITIAL');
 console.log('A:', aAsy()); // should be undefined
 console.log('B:', bAsy()); // should be undefined
}

In the shouldRedirect() case, we expect aAsy() to return "a" and bAsy() to return "b". However, the actual output shows aAsy() returning "b" and bAsy() returning "a". It's a classic mix-up! In the initial phase, both asynchronous operations should be undefined, as they haven't completed yet. This is the expected behavior when the component initially renders. However, the bug manifests when the component reruns due to a redirect or other state change. This rerun can happen because SolidJS is designed to be reactive and will re-execute components when their dependencies change. The problem is that during this rerun, the order in which the asynchronous operations are created might change, leading to the incorrect mapping of results. This is especially evident when the component logic includes conditional statements that alter the order of createAsync calls, as seen in our example. The key takeaway here is that the asynchronous data fetching process becomes unpredictable when the component's execution path varies between the initial render and subsequent reruns. This unpredictability is what leads to the mixed-up results, making it crucial to understand how SolidJS handles these scenarios and how we can mitigate the bug.

Code Snippet

Here's the relevant code snippet that triggers the issue:

if (shouldRedirect()) {
 // later
 // if we flip back the order it will become stable again
 aAsy = createAsync(() => a());
 bAsy = createAsync(() => b());
 // bAsy = createAsync(() => b());
 // aAsy = createAsync(() => a());
} else {
 // initial
 bAsy = createAsync(() => b());
 aAsy = createAsync(() => a());
}

In this snippet, the order of createAsync calls for aAsy and bAsy is flipped based on the shouldRedirect() condition. This flip is the root cause of the mixed-up results.

Why This Happens: The Root Cause

So, why does this happen? SolidJS reruns the suspended component. On the second run, it tries to match the results based on the original (initial) order of requests. Because we flip the createAsync order on the "later" phase, the results are matched incorrectly. This is a classic case of asynchronous race conditions, where the timing and order of operations can significantly impact the outcome. In our scenario, the order in which createAsync is called determines how SolidJS maps the asynchronous results. When the component reruns, it might expect the results in a different order than they were initially created, leading to the mix-up. The core issue here is that SolidJS, during server-side rendering, attempts to optimize the process by reusing the results of asynchronous operations. This optimization works well under normal circumstances but falls apart when the component's execution path changes. The component reruns and the resulting change in the order of createAsync calls break the expected mapping, causing the wrong data to be associated with the wrong resource. To visualize this, imagine a scenario where you have two promises, A and B. Initially, you create A and then B. SolidJS expects the results in this order. However, if the component reruns and you create B before A, the mapping goes awry because SolidJS is still expecting the result of A first. This is a simplified explanation, but it captures the essence of the problem. The interplay between component reruns, asynchronous data fetching, and the order of operations is what makes this bug tricky to debug and resolve.

Potential Solutions and Workarounds

Now that we understand the bug, what can we do about it? Here are a few potential solutions and workarounds:

1. Stabilize Execution Order

The most straightforward solution is to ensure that the order of createAsync calls remains consistent across reruns. In our example, we can achieve this by avoiding the conditional flipping of the order. One of the key strategies for addressing this issue is to stabilize the execution order of your asynchronous operations. This means ensuring that the calls to createAsync or createResource are made in a consistent order, regardless of how many times the component reruns. This consistency is crucial because SolidJS relies on the order of these calls to map the results correctly. If the order changes between renders, the mapping can break down, leading to the data mix-up we've been discussing. To achieve a stable execution order, you might need to refactor your component logic. Look for any conditional statements or branching paths that could potentially alter the order in which asynchronous operations are initiated. If you find such conditions, consider restructuring your code to ensure that the calls are always made in the same sequence. This might involve moving the createAsync calls outside of conditional blocks or using a different approach to manage the asynchronous data flow. For example, instead of conditionally creating resources, you could create them upfront and use signals to control when they are actually fetched. This way, the creation order remains consistent, even if the data fetching is triggered conditionally. In essence, stabilizing the execution order is about making your component's asynchronous behavior predictable. By ensuring that the calls to createAsync and createResource are always made in the same order, you can eliminate a major source of the bug and ensure that your server-side rendering works as expected. This approach might require some careful planning and restructuring, but it's often the most reliable way to address the issue.

2. Keying Resources

SolidJS provides a mechanism for keying resources, which can help the framework track them correctly across reruns. By providing a unique key, you can ensure that SolidJS doesn't mix up the results. Keying resources is another powerful technique to prevent the data mixing issue. SolidJS provides a mechanism to assign unique keys to resources, allowing the framework to track them correctly across reruns. By providing a key, you essentially give SolidJS a way to identify and differentiate between different asynchronous operations, even if their execution order changes. This can be particularly useful in scenarios where you have dynamic lists or conditional rendering of components that use createAsync or createResource. When a component reruns, SolidJS uses these keys to match the results of asynchronous operations to their corresponding resources. If a key is present, SolidJS can correctly associate the data with the resource, even if the order of calls has changed. However, if keys are missing or not unique, the framework might fall back to relying on the order of calls, which can lead to the mix-up we're trying to avoid. To implement keying, you can pass a key property within the options object when creating a resource. This key should be a stable value that uniquely identifies the resource. For example, if you're fetching data based on an ID, you could use the ID as the key. This ensures that the resource is always associated with the correct data, regardless of when and how it's created. The keying mechanism is especially effective when combined with other strategies, such as stabilizing the execution order. By using both approaches, you can create a robust system for managing asynchronous data in SolidJS, ensuring that your server-side rendering is both correct and efficient. It's important to carefully consider the keys you use, as they play a crucial role in the framework's ability to track and manage your resources effectively.

3. Conditional Rendering

Instead of flipping the order of createAsync, consider conditionally rendering different components or parts of the component. This can help avoid the secondary rerun issue altogether. This approach involves using SolidJS's conditional rendering capabilities to manage the different states of your component. Instead of changing the order of createAsync calls, you can conditionally render different parts of your component based on the state or conditions that trigger the bug. This can help you avoid the secondary reruns that cause the data mix-up in the first place. For example, in our example scenario, instead of flipping the order of createAsync calls based on the shouldRedirect() condition, we could render different components or sections of the component. One section would handle the initial state, and another would handle the redirected state. Each section would have its own set of createAsync calls, ensuring that the order remains consistent within each section. By isolating the asynchronous operations within these conditionally rendered sections, we can prevent the confusion that arises from the global reordering of calls. This approach also aligns well with SolidJS's component-based architecture, which encourages breaking down complex UIs into smaller, manageable pieces. By conditionally rendering these pieces, you can create a more modular and predictable application, making it easier to reason about and debug. Conditional rendering can also improve the performance of your application. By only rendering the necessary parts of the UI, you can reduce the amount of work that SolidJS needs to do, leading to faster rendering times and a more responsive user experience. However, it's important to use conditional rendering judiciously. Overusing it can lead to a fragmented and complex component structure. The key is to find the right balance between modularity and simplicity, ensuring that your components remain easy to understand and maintain.

4. Server-Side Data Serialization

Consider serializing the data fetched on the server and passing it to the client. This can help avoid redundant data fetching and ensure consistency between the server and client. Server-side data serialization is a powerful technique to ensure consistency between the data rendered on the server and the data used by the client-side application. This approach involves fetching the data on the server, serializing it (converting it into a string format), and then passing it to the client as part of the initial HTML. This way, the client-side application can use the serialized data to hydrate the initial UI, avoiding the need to refetch the data. Serialization can be particularly effective in scenarios where you're using server-side rendering (SSR) to improve the initial load time and SEO of your application. By pre-rendering the UI on the server and sending the data along with it, you can ensure that the user sees the content immediately, without having to wait for the client-side JavaScript to load and fetch the data. This can significantly enhance the user experience, especially on slower networks or devices. In the context of the bug we're discussing, serialization can help to circumvent the issue by ensuring that the data is fetched and processed in a controlled environment on the server. The serialized data acts as a snapshot of the state at the time of rendering, preventing the client-side application from encountering the mixed-up results caused by secondary reruns or asynchronous race conditions. There are various ways to implement server-side data serialization in SolidJS. One common approach is to use a library like devalue to serialize the data into a compact and efficient format. You can then embed this serialized data into the HTML as a JavaScript variable or use a <script> tag. On the client-side, you can deserialize the data and use it to initialize your application state. Serialization is not a silver bullet, and it might not be the best solution for every scenario. However, it's a valuable tool in the arsenal of a SolidJS developer, especially when dealing with server-side rendering and data consistency issues. By carefully considering when and how to serialize your data, you can create more robust and performant applications.

Conclusion

So, there you have it! We've explored a tricky bug in SolidJS and SolidStart that can cause createAsync and createResource to mix up results during server-side rendering. We've seen why this happens and discussed several strategies to work around it. Remember, stabilizing execution order, keying resources, conditional rendering, and server-side data serialization are your friends here. Keep these techniques in mind, and you'll be well-equipped to tackle this issue in your own projects. Thanks for diving into this with me, guys! If you have any questions or other solutions, feel free to share them below. Happy coding! By understanding the root causes and applying these solutions, you can build more reliable and performant SolidJS applications. Remember, the key is to ensure consistency and predictability in your asynchronous data fetching, especially in server-side rendering scenarios. Happy coding, and may your data always be in the right place!